Optical sensors for determining the moisture content of the soil in an irrigation system are well known. They usually take the form of a prism or similar structure, in which a light beam projected into the prism is internally reflected toward a light sensor, e.g. a photodiode. The amount of light received by the light sensor depends on the amount of moisture present at the surfaces of the prism. This moisture changes the optical characteristics of the prism surface and thereby causes a portion of the beam to be refracted outwardly of the prism, instead of being reflected inwardly toward the light sensor. The amount of refraction, and thus the amount of light received by the light sensor, translates into a measurement of the wetness of the soil.
Typical examples of the prior art are illustrated in U.S. Pat. No. 5,946,084 to Kubulins (a dome-shaped structure) and U.S. Pat. No. 6,079,433 to Saarem (a flat-surface prism). These devices are useful, but they are of limited efficiency because they rely on the reflection of divergent beams of light and are not readily suitable for miniaturization. Also, double-reflection devices of the prior art significantly limit the surface available for sensing and can only use a small arc of the illumination provided by the light source.